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Squawfish Population Viability Analysis --July 1993 Page 8 even more offspring as a larger individual in future years. (A mathematical demonstration of this idea, based on optimal control theory, is almost certainly impossible at this time, given the fuzzy data available to us.) The picture just drawn is one of large, long-lived adults with massive total lifetime outputs of eggs (a female view). But, since the population size is assumed to be stable, the recruitment of juveniles into the adult population must be extremely low. For example, if a reproductively mature female of seven years of age can be expected to live all additional seven years, she will only get a single daughter into this population of adults in that entire period of time. This calls into question mortality factors on juveniles. Very likely larger squawfish were the greatest predation threat to reproductively immature squawfish. This is consistent with the dispersal of juveniles to special nursery habitats free of the presence of adult squawfish. Juveniles then face several energy problems. They have to spend energy to disperse, and they also have to acquire energy to grow as fast as possible. Finally, they have to return upstream to spawn. (Genetic types that do not return upstream to spawn approximately where they hatch end up in the Sea of Cortez.) Thus, juveniles have the evolutionary incentive to delay the energy investment of reproduction until they could face life as a large adult squawfish. 1.7 Effects of Harvest on Age Structure Harvest by man, with or without a size or age bias, has both short-term ecological consequences and long-term evolutionary consequences. One evolutionary consequence is that the reproductive value curve for the Colorado squawfish immediately changes. Figure 1.2 shows the direction.